A major portion of the worldwide petrochemical industry is concerned with the production of light olefin materials and their subsequent use in the production of numerous important chemical products via polymerization, oligomerization, alkylation and the like well-known chemical reactions. Light olefins include ethylene, propylene and mixtures thereof. These light olefins are essential building blocks for the modern petrochemical and chemical industries. The major source for these materials in present day refining is the steam cracking of petroleum feeds. The art has long sought a source other than petroleum for the massive quantities of raw materials that are needed to supply the demand for these light olefin materials. A great deal of the prior art's attention has been focused on the possibility of using hydrocarbon oxygenates and more specifically methanol as a prime source of the necessary alternative feedstock. Oxygenates are particularly attractive because they can be produced from such widely available materials as coal, natural gas, recycled plastics, various carbon waste streams from industry and various products and by-products from the agricultural industry. The art of making methanol and other oxygenates from these types of raw materials is well established.
The art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products. These light olefin products must be available in quantities and purities such that they are interchangeable in downstream processing with the materials that are presently produced using petroleum sources. Although many oxygenates have been discussed in the prior art, the principal focus has been on methanol conversion technology. There are two major techniques for conversion of methanol to light olefins. The first of these MTO processes is based on early German and American work with a catalytic conversion zone containing a zeolitic type of catalyst system. U.S. Pat. No. 4,387,263 reports on a series of experiments with methanol conversion techniques using a ZSM-5-type of catalyst system wherein the problem of DME recycle is a major focus of the technology disclosed. Although good yields of ethylene and propylene were reported in this '263 patent, unfortunately they were accompanied by substantial formation of higher aliphatic and aromatic hydrocarbons which the patentees speculated might be useful as an engine fuel and specifically as a gasoline-type of material.
This early MTO work with a zeolitic catalyst system was then followed up by the Mobil Oil Company who also investigated the use of a zeolitic catalyst system like ZSM-5 for purposes of making light olefins.
Primarily because of an inability of this zeolitic MTO route to control the amounts of undesired C4+ hydrocarbon products produced by the ZSM-5 type of catalyst system, the art soon developed a second MTO conversion technology based on the use of a non-zeolitic molecular sieve catalytic material. This branch of the MTO art is perhaps best illustrated by reference to UOP's extensive work in this area as reported in numerous patents of which U.S. Pat. No. 5,095,163; U.S. Pat. No. 5,126,308 and U.S. Pat. No. 5,191,141 are representative. This second approach to MTO conversion technology was primarily based on using a catalyst system comprising a non-zeolitic molecular sieve, generally a metal aluminophosphate (ELAPO) and more specifically a silicoaluminophosphate molecular sieve (SAPO), with a strong preference for a SAPO species that is known as SAPO-34. This SAPO-34 material was found to have a very high selectivity for light olefins with a methanol feedstock and consequently very low selectivity for the undesired corresponding light paraffins and the heavier materials. This ELAPO catalyzed MTO approach is known to have at least the following advantages relative to the zeolitic catalyst route to light olefins: (1) greater yields of light olefins at equal quantities of methanol converted; (2) capability of direct recovery of polymer grade ethylene and propylene without the necessity of the use of extraordinary physical separation steps to separate ethylene and propylene from their corresponding paraffin analogs; (3) sharply limited production of by-products such as stabilized gasoline; (4) flexibility to adjust the product ethylene-to-propylene weight ratios over the range of 1.5:1 to 0.75:1 by minimal adjustment of the MTO conversion conditions; and (5) significantly less coke make in the MTO conversion zone relative to that experienced with the zeolitic catalyst system.
For various reasons well articulated in UOP's patents, U.S. Pat. No. 6,403,854; U.S. Pat. No. 6,166,282 and U.S. Pat. No. 5,744,680 (all of the teaching of which are hereby specifically incorporated by reference) the consensus of the practitioners in this OTO or MTO art points to the use of a fluidized reaction zone along with a fluidized regeneration zone as the preferred commercial solution to the problem of effectively and efficiently using an ELAPO or SAPO-type of catalyst system. As is well-understood by those of skill in the fluidization art, the use of this technology gives rise to a substantial problem of solid-vapor separation in order to efficiently separate the particles of the fluidized catalyst from the vapor products of the OTO or MTO reaction as well as from any unreacted oxygenate materials exiting the OTO or MTO conversion zone. Standard industry practice for accomplishing this difficult separation step involves its use of one or more vapor-solid cyclonic separating means which are well illustrated in the sole drawing of U.S. Pat. No. 6,166,282 where a series of three cyclonic separation means are used to separate spent OTO or MTO catalyst from the product effluent stream.
Despite the promising developments associated with the ELAPO or SAPO catalyzed routes to light olefins there are still substantial hurdles to overcome. Coking of surfaces within the reactor can reduce yield and productivity of these processes. Two particular potential coking problems are discussed herein. A coking problem that is the subject of this invention to resolve is the coking of surfaces as the result of reactive materials remaining in stagnant zones within the reactor. A second coking problem can be the result of recycling of unreacted oxygenate together with recycling of various reaction by-products combined with the oxygenate feed stream.